Conventionally, in a brake apparatus that obtains braking force by pressing of a friction member using a motor, a relationship between a stroke of an actuator and rigidity thereof is nonlinear. Accordingly, in the case of increasing braking force at a constant proportion, a speed of the motor becomes high during an initial period of brake operation, and low during a final period thereof. In this case, it is not possible to use an intermediate speed in which efficiency of the motor is the best, and thus improvement in motor responsiveness is impaired. Further, it is not possible to set a reduction ratio of the motor so as to simultaneously increase responsiveness and efficiency. Thus, the reduction ratio must be set to a moderate value so that improvement in motor efficiency and reduction in electric power consumption are hindered.
To address this, an electronic brake using a ball ramp mechanism has been disclosed in which a tilting surface of the ball ramp mechanism for reducing electric power consumption is formed from a plurality of tilting portions each of which has different tilting angle (Japanese Patent Laid-open No. 2001-41269).
However, in the electronic brake, the titling angle of the tilting surface is changed, and thus it is not possible to dispose a plurality of balls at the tilting surface, and as a result, load is supported by a single ball. Therefore, a surface pressure of a point contact portion of the ball and the tilting surface is extremely large, and motor efficiency decreases because energy for operating the motor is lost.